In recent years, from the viewpoint of global environmental conservation, energy saving with a lubricating oil and the extended service life of a lubricating oil are important problems. In order to solve the problems, it is essential to improve the performance including the viscosity index improving capability, the low-temperature viscosity characteristics, the pour point depressing capability, the shear stability, and others of a lubricating oil additive containing a polymer and being added to a lubricating oil.
Lubricating oil additives containing polymers are classified through the molecular structures of the polymers. For example, they are classified into an acrylic lubricating oil additive, an olefinic lubricating oil additive, an isobutylenic lubricating oil additive, a styrene diene copolymeric lubricating oil additive, and others. Among those, the acrylic lubricating oil additive is excellent in viscosity index improving capability, low-temperature viscosity characteristics, and pour point depressing capability, and hence is widely used for an engine oil (for a gasoline engine or a diesel engine), a drive line oil [a gear oil (a manual transmission oil, a differential oil, or the like), an automatic transmission oil (ATF (Automatic Transmission Fluid), CVTF (Continuously Variable Transmission Fluid), or the like), or the like], a hydraulic oil (a power steering oil, a shock absorber oil, or the like), or the like.
As an acrylic polymer used for an acrylic lubricating oil additive, an acrylic polymer that is synthesized mainly by a radical polymerization method and has a wide molecular weight distribution has heretofore been known. For example, in a polymer having such a wide molecular weight distribution that the molecular weight distribution is 2.2 to 3.4 (refer to Patent Reference 1), the proportion of the high molecular weight polymer is high and the macromolecular chains are likely to be cut by shearing in comparison with a similar polymer having a narrow molecular weight distribution. The problems have therefore been that the shear stability is low and the service life of a lubricating oil is short. Further, although the shear stability is improved by using a polymer having a low molecular weight in some cases, in such a case the thickening effect deteriorates and it has been necessary to increase the addition amount of the polymer.
In order to solve the problems, a lubricating oil additive comprising an acrylic polymer and having a narrow molecular weight distribution is studied. The examples are as follows:
(1) a viscosity index improver comprising an acrylic polymer having a narrow molecular weight distribution, the viscosity index improver being produced by a radical polymerization method of using a specific chain transfer agent (refer to Patent Reference 2);
(2) a lubricating oil additive comprising an astrally branched acrylic polymer having a narrow molecular weight distribution, the lubricating oil additive being produced by anionic polymerization (refer to Patent Reference 3); and
(3) a pour point depressant comprising an acrylic polymer having a narrow molecular weight distribution, the pour point depressant being produced by anionic polymerization (refer to Patent Reference 4).    Patent Reference 1: U.S. Pat. No. 6,271,184    Patent Reference 2: JP-A No. 48987/1997    Patent Reference 3: U.S. Pat. No. 6,013,735    Patent Reference 4: U.S. Pat. No. 5,834,408
In the case of the above item (1), evaluation is made on the viscosity index improver comprising an acrylic random copolymer having a molecular weight distribution of 1.4 to 2.0. However, the effect of reducing the addition amount required for obtaining the same dynamic viscosity is insufficient.
Further, in the case of the above item (2), the use of an acrylic star polymer having a molecular weight distribution of 1.3 to 1.8 as a lubricating oil composition is described. However, the degree of the improvement of the viscosity index is not obvious and the above problems are not solved.
Furthermore, in the case of the above item (3), an example wherein an acrylic copolymer having a molecular weight distribution of about 1.2 to 1.4 is used as a pour point depressant is described. However, the performance is not satisfactory.